A: The minute the public beta opens, invite your
friends. Spread the word via Twitter, blogs, and email far and wide.
Hang out on the site answering incoming questions during the first few
hours so that newcomers understand what a great experience a working,
popular Stack Exchange site can be.

(Presumably, "invite" here means "encourage your friends to join" rather than "send them an invitation")

But the private beta website has

Invite Fellow Experts

email a private beta invite to:

message (optional):

Send invite

Perhaps inviting experts to the private beta is a new feature, and the FAQ about private beta hasn't been updated.